Naomi Osaka and Karolina Pliskova were both victorious at the Rogers Cup in Toronto as the race to become the world No 1 on Monday takes shape.
Ashleigh Barty's shock first-round loss to Sofia Kenin opened the door for either Osaka or Pliskova to leave Canada with top spot.
Osaka's victory on Wednesday guaranteed that the No 1 ranking would switch hands at the tournament's conclusion and now all eyes will be on the duo's results.
Osaka eased into the third round after taking the opening set 6-2 against Tatjana Maria before the qualifier was forced to retire.
Maria wasn't able to cope with an abdominal injury but the opening 33 minutes indicated that Osaka would have prevailed anyway if the match had gone the distance.
Osaka has recently spoken about her 'rough' 2019 so far. This was her first match since losing in the first round of Wimbledon and after the victory, the 21-year-old was content with her work.
"I thought I played really well despite the fact that we only played one set," said the second seed.
"I think that I was able to do what I wanted, which was go for the shots that I had the opportunity to. For me, I'm just happy that it was my first match in a while and I was able to get in the groove of things quickly."
Osaka will now encounter Iga Swiatek in the Round of 16 after the Polish player overturned a poor start to beat the 15th seed Caroline Wozniacki.
Swiatek's 1-6 6-3 6-4 victory was the second Top 20 win of her career and the 18-year-old showed considerable maturity to battled back from such a tough first set.
"It's going to be exciting," said Swiatek about her clash with Osaka.
"Naomi is a great player, already a legend. I just want to do my best and give a show. Even if I lose, I hope to show my best tennis."
Pliskova is seeded third in Toronto and managed to navigate her way past Alison Riske 6-4 6-7 (7-4) 6-2.
The Czech player countered nine double faults with 11 aces and saved six of the seven break points that Riske created against her.
Pliskova knows she will have to go deep into the tournament if she is to take the world No 1 spot and faces Anett Kontaveit next up.
Kontaveit was another who spent less time on court than expected after Carla Suárez Navarro retired midway during their second set.
The Estonian was leading 7-5 3-1 at the time and will have welcomed the greater rest after a two-hour and 41-minute clash with Maria Sharapova in the first round.
Serena Williams is seeded eighth in Toronto and fought back from 3-1 down in the first set and 2-0 in the second, to deliver a 6-3 6-3 victory over Elise Mertens.
Serena has won the Rogers Cup four times previously and is playing in Toronto for the first time in four years.
"I feel like my movement is great - I've been working on my fitness, so I felt like it really was able to shine through," Williams said. "I'm loving going out there, and I'm loving kind of running. So it's a good thing."
Russian qualifier Ekaterina Alexandrova will be the next player to duel with Serena and it will be their first-ever career meeting.
Elsewhere in the draw, Bianca Andreescu played out a 5-7 6-2 7-5 thriller against Daria Kasatkina.
Andreescu, who is returning from a shoulder injury, found herself 5-3 down in the deciding set before notching up four games in succession.
"In those moments, I just let loose, really. I know when I do that, I play my best tennis," the Canadian said after her victory.
"It's when I just stay relaxed and I go for my shots. That's what I did towards the end, and I think that's what helped me win. I felt that she was holding back a lot, and I was just on top of everything."
Kiki Bertens will be Andreescu's opponent in the Round of 16 and they will be second on Centre Court on Thursday as 16 players are cut to eight and the quarter-finals are decided.